It is known from EP-A-1992876 to feed a boiler selectively with air or an oxygen rich gas. The boiler is supplied with fuel, which may be biomass fuel or a fossil fuel such as coal or natural gas. The boiler may for example be of the pulverized fuel (known for coal as PC) or fluidized bed or the cyclone type.
The advantages of combustion in a boiler fed with oxygen rich gas include:                improved combustion, especially when the fuel has a reduced calorific value,        improved thermal efficiency of the boiler due to reduced heat loss in the flue gas,        higher flame temperature, allowing the exchange surface to produce more steam,        reduced NOx emissions (as described in U.S. Pat. No. 6,568,185),        improved throughput for the boiler.        
However, oxygen enriched gas is not generally sent to boilers because of the cost of the oxygen rich gas.
It is; however, well known to send oxygen rich gas to a unit consuming oxygen rich gas in large quantities, the unit being for example integrated in a unit for the generation of electricity. For example, an oxycombustion boiler is generally fed with oxygen rich gas and fuel. The combustion is used to produce heat which in turn raises steam to be used in a steam turbine to generate electricity. This type of unit produces a carbon dioxide containing waste gas which is at least in part recycled to the unit, possibly after treatment. The first unit may also for example be an IGCC unit or a gasification unit, such as a coal gasification unit.